This invention relates to an adhesive for printed wiring boards produced by an additive process excellent in adhesiveness to copper deposited by electroless plating, and a process for producing the printed wiring boards.
According to the additive process, a printed wiring board has been produced by forming necessary wiring patterns by electroless plating on an insulating substrate having an adhesive layer thereon. For example, an adhesive layer containing a catalyst is formed on an insulating substrate containing a catalyst or the like by an immersion method or a curtain coating and cured by heating at 150.degree. to 190.degree. C. for 30 to 60 minutes. Then, portions other than circuit forming portions are masked with a plating resist, and subjected to chemical roughening on the circuit forming portion surface of adhesive layer selectively with an oxidative etching solution such as chromic acid-sulfuric acid as a pre-treatment for improving the adhesiveness to electrolessly plated copper. After neutralization and water washing steps, the thus treated substrate is immersed in an electroless copper plating solution to deposit copper on the circuit portions. Thus a wiring pattern is formed.
As adhesives for printed wiring boards produced by the additive process, it is known to use as a major component acrylonitrile-butadiene rubber which is generally good in adhesiveness to plated copper, to crosslink the acrylonitrile-butadiene rubber with an alkylphenol resin so as to ensure heat resistance, and to mix with an epoxy resin so as to ensure electrical properties as disclosed in Japanese Paten Examined Publication (JP-B) Nos. 45-9843, 55-16391, H1-53910, Japanese Patent Unexamined Publication (JP-A) No. 48-24250 etc. Further, in order to reinforce an adhesive coated film and to increase roughness of adhesive surface at the time of chemical roughening (for improving adhesiveness to plated copper by an anchoring effect), it is proposed to blend an inorganic filler with the adhesive.
On the other hand, as a process for producing a laminate having an adhesive thereon, a process for forming an adhesive layer on an insulating substrate simultaneously at the time of molding the insulating substrate (hereinafter referred to as "one step method") is reconsidered recently comparing with a known process of using an insulating substrate as a starting material from the viewpoints of production cost, uniform film thickness, low surface roughness, and the like.
With recent miniaturization and multi-function of electronic devices, printed wiring boards are required to have an increased wiring density. Thus, the reduction of wiring width proceeds rapidly. For example, the wiring width of 150 .mu.m was mainly 2 or 3 years ago, but now the wiring width of 100 .mu.m becomes mainly. In the future, the wiring width will be 75 .mu.m or 50 .mu.m.
Under these circumstances, in printed wiring boards produced by the additive process according to either a conventional method or the one step method, the adhesiveness to plated copper becomes very important. But when the one step method is used, there takes place a phenomenon so-call "migration" wherein at the time of molding a prepreg containing a thermosetting resin and the adhesive into one body, the thermosetting resin contained in the prepreg is mixed at the time of pressing with heating. The migration brings about lowering of adhesive strength between the adhesive and electrolessly plated copper. Therefore, it is usually employed a method of carrying out curing of the adhesive sheet previously and pressed to the prepreg with heating. But, since curing proceeds considerably, the roughening becomes difficult, and the adhesive strength between the adhesive and electrolessly plated copper is lowered to about 70 to 80% of that of a conventional case of using the laminate having an adhesive thereon as a starting material.